SARS-CoV-2 Seroprevalence in a Cohort of Asymptomatic, RT-PCR Negative Croatian First League Football Players

Background: During the COVID-19 pandemic the Croatian Football Federation has launched a new model of pre-season systematic examination of football players, emphasizing the diagnosis of asymptomatic SARS-CoV-2 infection and preventing further spread among the players. Objectives: The aim of this study was to assess the prevalence and dynamics of SARS-CoV-2 IgA and IgG antibodies in the cohort of asymptomatic and SARS-CoV-2 PCR negative professional football players in the Croatian First Football League by using a commercial ELISA antibody assay in the paired serum samples taken 2 months apart. Methods: Serology testing was performed from May till July 2020 in a cohort of 305 asymptomatic football players and club staff members. RT-PCR for detection of SARS-CoV-2 from nasopharyngeal swabs was performed on three occasions, and Euroimmun ELISA for detection of IgA and IgG (S1 and NCP) antibodies was tested in paired serum samples in May and July. Results: All RT-PCR results were negative. Sixty-one (20%) participants were reactive in one or two classes of antibodies at baseline and/or follow-up serology testing. IgA reactivity was found in 41 (13.4% [95% CI=10.7-17.7]) baseline sera and 42 (13.8% [95% CI=10.3-18.9]) follow-up sera. IgG to S1 protein was found in 6 (2% [95% CI=0.9-4.2]) participants at baseline and 1 (0.33% [95% CI=0.0006-1.83]) at follow-up. IgG to NCP was found in 2 (0.7% [95% CI=0.2-2.4]) participants at baseline and 8 (2.6% (95% CI=1.3-5.1]) participants at follow-up. Noticeable dynamics in the paired sera was observed in 18 (5.9%) participants (excluding borderline IgA results) or 32 (10.5%) (including IgA borderline results). Conclusion: Various patterns of IgA and IgG reactivity were found in the paired serum samples. Based on serology dynamics we estimate that in 5.9%-10.5% of PCR negative football players asymptomatic exposure to SARS-CoV-2 during pandemics could not be excluded.